Two processes are known in the art for the preparation of caprolactam from butadiene. The first process involves the following steps: (1) stepwise addition of two equivalents of hydrogen cyanide to butadiene to produce adiponitrile, (2) partial hydrogenation of adiponitrile to form 6-aminocapronitrile, (3) separation of 6-aminocapronitrile from fully hydrogenated hexamethylenediamine and unreacted adiponitrile, and (4) hydrolysis of 6-aminocapronitrile and cyclization of the hydrolysis product to produce caprolactam.
The second process involves the following steps: (1) carbonylation of butadiene to methyl 3-pentenoate, (2) hydroformylation of methyl 3-pentenoate to methyl 5-formylvalerate, (3) reductive amination of methyl 5-formylvalerate to methyl 6-aminocaproate, and (4) cyclization of methyl 6-aminocaproate to caprolactam.
It would be desirable to develop a process that produces fewer by-products than the above processes and involves fewer reaction steps. A process that satisfies these needs could be based on the carbonylation of 3-pentenenitrile to 5-cyanovaleric acid or its corresponding esters. The process to produce caprolactam would involve the steps of: (1) hydrocyanation of butadiene to 3-pentenenitrile, (2) carbonylation of 3-pentenenitrile to 5-cyanovaleric acid or ester, and (3) hydrogenation and cyclization of 5-cyanovaleric acid or ester to caprolactam.
In order to develop a successful process, an active, selective catalyst that operates under mild conditions for the carbonylation of 3-pentenenitrile is needed. Previous attempts to produce methyl 5-cyanovalerate or 5-cyanovaleric acid from 3-pentenenitrile were based on the use of a cobalt catalyst. U.S. Pat. No. 5,434,290 discloses the use of a cobalt catalyst, an activating solvent comprising carbonic diesters, carbamates or ureas and CO pressures between 210 to 270 bar to convert 3-pentenenitrile to methyl 5-cyanovalerate. U.S. Pat. No. 4,508,660 discloses a similar process, but using sulfones as the preferred solvent and CO pressures between 14 to 35 MPa. The rates of carbonylation reported in this patent are quite low, the turnover frequency calculated for example 2 of U.S. Pat. No. 4,508,660 gives 1.52 mol/mol-hour. A similar situation is described in U.S. Pat. No. 4,933,483.
Palladium-based catalysts for the carbonylation of olefins and diolefins are known in the art. U.S. Pat. No. 5,028,734 discloses a process for the selective carbonylation of conjugated dienes in the presence of an alcohol and a catalyst system comprising a halide-free palladium salt, a bidentate phosphine ligand and a protonic acid with a pKa value greater than 3. PCT patent application WO 00/56695 discloses a process for the carbonylation of conjugated dienes by reaction with CO and alcohol in the presence of a catalyst system including a source of palladium cations, a phosphorus-containing ligand of structure X1—R—X2 and a source of anions. The preferred ligands are based on a 9-phosphabicyclononyl group for X1 and X2 and a simple bridge for R. PCT patent application WO97/38964 describes the use of the same catalyst system for the carbonylation of ethylenically unsaturated compounds.
PCT patent application WO 98/42717 describes a catalyst of the formula R1>P—R2—PR3R4 for the carbonylation of terminal and internal olefins. The R1>P moiety is a substituted 2-phosphatricyclo [3.3.1.1{3,7}]decyl group where one or more of the carbon atoms are replaced by heteroatoms, in particular oxygen. Comparisons are made between phosphines such as (Me3C)P(CH2) 3P(CMe3) (DTBPP) and 1,3-P,P′-di(2-phospha-1,3,5,7-tetramethyl-6,9,10-trioxatricyclo[3.3.11{3.7}]decyl propane (DPA3). Carbomethoxylation of an internal-C14 olefin feed with a palladium-based catalyst using DPA3 as a ligand gives linear methyl ester with 78% selectivity at an average rate of 120 mol/mol-hour. In contrast, using DTBPP as ligand under identical conditions gives only an average rate of 5 mol/mol-hour.
PCT patent application WO96/19434 and Chem Comm., 1999, 20, 1877–1878 describe the use of bidentate phosphines such as bis(di-t-butyl phosphinp)-o-xylene for the carbonylation of ethylene to methyl propanoate. The patent application describes catalysts using these bidentate phosphines as being unable to carbonylate propene. One skilled in the art might reason that if such catalysts are able to carbonylate ethylene, but fail to carbonylate propene at any appreciable rate, then these same catalysts would be inactive for the carbonylation of internal olefins. Surprisingly, these catalysts are able to convert an internal olefin, namely 3-pentenenitrile, to the corresponding linear carboxylic acid, namely 5-cyanovaleric acid (or its alkyl esters). Additionally these catalysts are able to convert 3-pentenoic acid to adipic acid, and to convert methyl 3-pentenoate to dimethyl adipate.